The proposed invention relates to UMTS 3rd Generation (3G) wireless communications. More specifically, it considers the Time Division Duplex (TDD) mode of operation using dynamic link adaptation (DLA).
A variety of services, such as video, voice and data, each having different Quality of Service (QoS) requirements, can be transmitted using a single wireless connection. This is accomplished by multiplexing several transport channels onto a coded composite transport channel (CCTrCh). The CCTrCH is then mapped onto physical channels for transport over the air interface. Each transport channel is associated with a transport format set (TFS), which defines a set of allowed transport formats (TF). Parameters such as transport block size and transport block set size are considered dynamic since they can vary within a TFS. In contrast, semi-static parameters cannot be dynamically changed for a given transport channel. Rather, they can only be changed after Radio Resource Control (RRC) signaling has been exchanged between the user equipment (UE) and the UMTS Terrestrial Radio Access Network (UTRAN). The time expenditure of this exchange to adjust semi-static parameters can have unacceptable consequences with respect to timely mitigation of an RF propagation failure.
Forward error correction (FEC) coding type and rate are semi-static parameters that are identical for each TF within a TFS. An FEC coding rate of ½ indicates roughly 2 times as many bits are required to transmit 1 bit of information, while a ⅓ rate means there are about 3 times as many bits. A coding rate of ½ allows one extra FEC bit to be added for each data bit. For coding rate ⅓, two extra FEC bits are added for each data bit. This allows the timeslot to tolerate a lower SIR.
There are a variety of possible combinations when multiplexing several transport channels onto a CCTrCh. A particular transport format combination (TFC) specifies the transport format of each of the multiplexed channels. A TFC set is a set of allowed TFCs.
A transport format combination indicator (TFCI) is an indicator of a particular TFC, and is transmitted to the receiver to inform the receiver which transport channels are active for the current frame. The receiver, based on the reception of the TFCIs, will be able to interpret which physical channels and which timeslots have been used. Accordingly, the TFCI is the vehicle which provides coordination between the transmitter and the receiver such that the receiver knows which physical transport channels have been used.
FIG. 1A shows a UTRA protocol stack, which contains the following lower layers: radio link control (RLC), medium access control (MAC) and physical (PHY).
The RLC layer delivers logical channels bearing control information to the MAC layer. These channels are the dynamic control channel (DCCH), which includes set-up information, and the dynamic traffic channel (DTCH), which carries user data such as voice and data.
The MAC layer maps the logical channels DCCH and DTCH to different transport channels (DCHs), which are then delivered to the PHY layer. The MAC layer is responsible for selecting the TFC for combination of transport channels DCH within the CCTrCH. This selection occurs at every transmission time interval (TTI), which is the period of time for one data burst. For example, a 20 ms TTI represents a transmittal of data specified in the TF every 20 ms (typically amounting to two 10 ms frames). Typically, there are 15 timeslots in each frame. The TFC selection is based on the amount of buffered data of each logical channel and the UE transmission power on the uplink (UL) communication. The TFC defines all of the dynamic and semi-static parameters for each transport channel within the CCTrCH. The selected TFC and associated data for each UL CCTrCH is provided to the physical layer for transmission. If the physical layer subsequently determines transmission of this TFC exceeds the maximum or allowable UE transmission power, a physical status indication primitive is generated to the MAC to indicate that maximum power or allowable transmission power has been reached.
FIG. 1B shows a block diagram of the PHY layer combining transport channels DCH_A, DCH_B and DCH_C on the CCTrCH and mapping them into physical channels for transmission over the air interface. A data burst occurs as one coded packet of data is mapped in one time slot on the physical channel. The PHY layer is responsible for performing the channel coding of transport channels DCH, including any forward error correction (FEC). Among the parameters contained in the TFC are the defined FEC coding types and rates. The system chooses, on a TTI basis, which transport channels will be active and how much data will be transmitted in each one. That is, the TFC selection is fixed for the duration of the TTI, and can only be changed at the commencement of the next TTI period. The TFC selection process takes into account the physical transmission difficulties, (maximum allowable power being one), and reduces the physical transmission requirements for some time duration.
After the multiple transport channels are combined into a single CCTrCh, the CCTrCh is then segmented and those segments are mapped separately onto a number of physical channels. In TDD systems, the physical channels may exist in one, or a plurality of different timeslots, and may utilize a plurality of different codes in each timeslot. Although there are as many as 16 possible codes in a timeslot in the downlink, it is more typical to have, for example, 8 codes in a particular downlink in a particular timeslot. A connection can be assigned as many as 16 codes in a downlink timeslot. In the UL, the UE is limited to using two codes in any particular timeslot. There are a number of physical channels defined by a plurality of codes in a plurality of timeslots. The number of physical channels assigned per connection can vary.
In the UL, there are rarely more than two codes in a particular timeslot. In any event, there are a number of physical channels defined by a plurality of codes in a plurality of timeslots. The number of physical channels can vary.
Dynamic link adaptation (DLA) is a fast adjustment mechanism performed by the UE to combat difficult RF propagation conditions. When a UE reaches its maximum transmission power, it can reduce its data rate, typically by ½, in an attempt to correct signal to interference ratio (SIR), by restricting its TFC set to combinations having lower power requirements. For example, in a simple case having a single transport channel, and the TFC corresponding to the allowed transport formats of the transport channel DCH, such a transport channel may support data rates of 0, 16, 32, 64, and 128 kbps. In this example the TFC set would be (TF0, TF1, TF2, TF3, TF4), where TF0=0 kbps, TF1=16 kbps, TF2=32 kbps, TF3=64 kbps, TF4=128 kbps. Since transmitting at a higher data rate requires more power, the data rate is limited during times of congestion by restricting the TFC set to (TF0, TF1, TF2, TF3). This eliminates the possibility of the higher data rate TF4 being used. Blocked TFCs may be later restored to the set of available TFCs by unblocking them in subsequent periods when the UE transmission power measurements indicate the ability to support these TFCs with less than or equal to the maximum or allowed UE transmission power.
In the 3GPP UTRAN TDD standard, it is specified that physical resources (i.e., data) must be assigned in the PHY layer in sequential order, first by timeslot and then by code. Thus, during each data burst, the first code of the first timeslot is assigned, then the second code of the first timeslot and so on until the first timeslot is completely assigned. The assignment of data continues with the first code of the next consecutive timeslot, the second code of that timeslot, and so on for the necessary number of available timeslots and codes until data resource requirements are satisfied. Upon degraded RF conditions, DLA decreases the data rate and hence reduces the amount of required physical resources per TTI. However, the UE assigns physical resources to timeslots within the frame in consecutive order, regardless of RF conditions for a particular timeslot. As a result, if the first few timeslots are the ones having poor SIR, the later timeslots with potentially more favorable RF conditions are not utilized or underutilized.